Polyolefin microporous membrane and method of producing the same, separator for non-aqueous secondary battery and non-aqueous secondary battery

ABSTRACT

The present invention provides a polyolefin microporous membrane in which a degree of crystallinity is from 60 to 85%, and a tie molecular volume fraction is from 0.7 to 1.7%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2011/057247, filed on Mar. 24, 2011 (claiming priority fromJapanese Patent Application Nos. 2010-068117, 2010-068118, and2010-068119, all filed on Mar. 24, 2010), the contents of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a polyolefin microporous membrane, andparticularly to a technique for improving the safety and properties of anon-aqueous secondary battery.

BACKGROUND ART

A non-aqueous secondary battery represented by a lithium ion secondarybattery which uses, as a positive electrode, lithium-containingtransition metal oxides as represented by lithium cobaltate and uses, asa negative electrode, carbon material which is capable of doping anddedoping lithium is characterized by having a high energy density. Bythis characteristic, a non-aqueous secondary battery is important as abattery for portable electronic appliances represented by a cellularphone. Along with the fast popularization of these portable electronicappliances, demand therefor is ever-increasing.

Large numbers of vehicles which are conscious of environmentalresponsiveness, such as hybrid vehicles, are being developed. As a powersource mounted on a vehicle, a lithium ion secondary battery having ahigh energy density is attracting a great deal of attentions.

Most lithium ion secondary batteries are composed of a layered body of apositive electrode, a separator containing electrolyte and a negativeelectrode. A principal function of the separator is to prevent shortcircuit between a positive electrode and a negative electrode, andexamples of required properties of the separator include permeability oflithium ion, strength and durability.

At present, as a film suitable for a separator for lithium ion secondarybattery, a large number of varieties of polyolefin microporous membranesare proposed. A polyolefin microporous membrane satisfies theabove-mentioned required properties and has, as a safety function athigh temperature, a so-called shutdown function, which is a thermalrunway preventing function by shutting down the current by blockingholes due to a high temperature. Therefore, a polyolefin microporousmembrane is widely used for a separator for lithium ion secondarybattery.

However, there are cases where the temperature inside the batteryexceeds the melting point of polyethylene constituting a microporousmembrane, even when the shutdown function works and the holes ofpolyethylene microporous membranes are blocked whereby the electriccurrent is temporarily shutdown. When the limit of the heat resistanceof the polyolefin microporous membrane is exceeded, the microporousmembrane per se melts, and the shutdown function is lost. As a result, ashort circuit between electrodes triggers heat runaway of the battery.At this point, a breaking in a device in which the lithium ion batteryis installed or an accident due to ignition may occur. For this reason,in order to ensure further safety, a separator which can maintain theshutdown function even at a high temperature is demanded.

Therefore, in Patent Document 1, proposed is a separator for non-aqueoussecondary battery in which the surface of a polyethylene microporousmembrane is covered with a heat-resistant porous layer composed of heatresistant polymer such as fully aromatic polyamides. In Patent Document2, disclosed is a configuration in which inorganic particulates such asalumina are contained in a heat-resistant porous layer, to therebyimprove a heat resistance as well as a shutdown function. In PatentDocument 3, disclosed is a configuration in which metal hydroxideparticulates such as aluminium hydroxides are contained in aheat-resistant porous layer, to thereby improve a flame resistance aswell as a shutdown function and a heat resistance. In theseconfigurations, excellent effects can be expected from the viewpoint ofthe safety of the batteries on this point that the shutdown function andthe heat resistance are gone together.

PATENT DOCUMENTS

-   Patent Document 1 JP 2005-209570 A-   Patent Document 2 WO 2008/062727 A1-   Patent Document 3 WO 2008/156033 A1

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, the separator for non-aqueous secondary battery has a structurein which the polyolefin microporous membrane is coated with aheat-resistant porous layer. By this, the shutdown function whichpolyolefin microporous membrane has tends to be restrained. Therefore,the polyolefin microporous membrane has been required to have a highshutdown function. However, when the composition of the polyolefinmicroporous membrane is made such that flowability of the polyolefin ishigh in order to improve the shutdown function of the polyolefinmicroporous membrane, the mechanical strength of the polyolefinmicroporous membrane decreases. As a result, there has been a problemthat the mechanical strength of the separator for non-aqueous secondarybattery decreases.

Recently, from the viewpoint of making the capacity of lithium ionbattery high, a variety of high-capacity type positive electrodematerials and negative electrode materials are developed. In suchhigh-capacity type positive and negative electrode materials, there aremany cases that volume change during charge and discharge is large.Therefore, a problem arises in which the battery properties decreasewhen a large volume change of the electrode occurs.

In other words, the separator is disposed between the positive electrodeand the negative electrode. When charge and discharge of the battery areperformed, a compressive force or a restoring force is applied in thethickness direction of the separator due to the expansion and shrink ofthe electrode. In the case of low-capacity type positive and negativeelectrode materials such as conventional lithium cobaltate or hardcarbon, the volume change of the electrode is small. Therefore, thedeformation of the separator in the thickness direction is small, thebattery properties are not particularly affected. However, in the caseof using an electrode material which has a large volume change duringcharge and discharge such as high-capacity type positive and negativeelectrode materials, the acting force which of the electrode to theseparator becomes large. Subsequently, there are cases where theseparator cannot follow the volume change of the electrode. the porousstructure of the separator cannot recover from a compressed state, aphenomenon in which a sufficient amount of electrolyte cannot beretained in the holes of the separator, that is, a liquid depletionphenomenon may occur. This liquid depletion phenomenon may consequentlydeteriorate the repeated charge-discharge property (cycle property) ofthe battery.

In order to solve the liquid depletion problem, it is conceivable thatthe physical properties such as elasticity of the polyolefin microporousmembrane is controlled. As mentioned above, good shutdown properties andmechanical strength are also demanded for the polyolefin microporousmembrane, and when a certain physical property of the polyolefinmicroporous membrane is controlled, other physical properties arenecessarily also affected. Therefore, a technique is desired in whichthese various properties can be improved with balance.

The present invention is devised in view of the above circumstances.Under the above mentioned circumstances, a polyolefin microporousmembrane in which, even when the polyolefin microporous membrane iscomplexed with a heat-resistant porous layer, good mechanical strengthand shutdown properties can be obtained and electrolyte depletion isprevented, and a method of producing the same, a separator fornon-aqueous secondary battery and a non-aqueous secondary battery areneeded to be provided.

Means for Solving the Problems

In order to solve the above problems, the present inventors intensivelystudied to discover that the above problems can be solved by thefollowing constitutions.

The present invention is a polyolefin microporous membrane which has adegree of crystallinity of 60 to 85%, and a tie molecular volumefraction of from 0.7 to 1.7%.

The present invention is a separator for non-aqueous secondary batterywhich includes the polyolefin microporous membrane and a heat-resistantporous layer(s) containing heat resistant resin provided on one side orboth sides of the polyolefin microporous membrane. The present inventionis a separator for non-aqueous secondary battery which includes thepolyolefin microporous membrane and an adhesive porous layer(s)containing vinylidene fluoride resin provided on one side or both sidesof the polyolefin microporous membrane.

The present invention is a non-aqueous secondary battery, which includesa positive electrode, a negative electrode and the polyolefinmicroporous membrane or the separator for non-aqueous secondary batterywhich is disposed between the positive electrode and the negativeelectrode, and wherein an electromotive force is obtained by doping anddedoping lithium.

The present invention is a method of producing a polyolefin microporousmembrane, including preparing a polyolefin solution by melt-kneadingfrom 1 to 35 parts by mass of polyolefin and from 65 to 99 parts by massof mixed solvent composed of a volatile solvent and a nonvolatilesolvent at a temperature of from 190 to 220° C.; forming a gelcomposition by extruding the polyolefin solution through a die at atemperature from the melting point of the polyolefin to the meltingpoint +60° C. and by cooling the extruded polyolefin solution; removingthe volatile solvent from the gel composition; drawing the gelcomposition; and removing the nonvolatile solvent from the gelcomposition.

Effects of the Invention

By the present invention, a polyolefin microporous membrane in whicheven when the polyolefin microporous membrane is complexed with aheat-resistant porous layer, good mechanical strength and shutdownproperties are obtained and electrolyte depletion is prevented, a methodof producing the same, and a separator for non-aqueous secondary batterycan be provided.

By the present invention, a non-aqueous secondary battery in which thesafety and battery properties are improved can be provided.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, embodiments of the present invention is describedsequentially. These description and Examples illustrate the presentinvention and the scope of the present invention is not limited thereto.

[Polyolefin Microporous Membrane]

The term “microporous membrane” as in “polyolefin microporous membraneof the present invention”, means a membrane which has a large number ofmicropores inside and has a structure in which these micropores areconnected to each other, wherein gases or liquids can pass from one sideof the membrane to the other side of the membrane.

Examples of the raw materials of polyolefin microporous membrane of thepresent invention can include polyolefins such as polyethylene,polypropylene, polymethylpentene and copolymer thereof. Among these,polyethylene is preferred, and a high-density polyethylene or a mixtureof a high-density polyethylene and an ultra-high molecular weightpolyethylene is more preferred, from the viewpoint of the strength, theheat resistance or the like.

In the case of polyethylene, a weight-averaged molecular weight of500,000 to 5,000,000 is suitable, and a polyethylene composition having1% by mass or higher of an ultra-high molecular weight polyethylenehaving a weight-averaged molecular weight of 1,000,000 or higher isparticularly preferred. Further, a polyethylene composition having 10 to90% by mass of ultra-high molecular weight polyethylene having aweight-averaged molecular weight of 1,000,000 or higher is suitable.

The density of the high-density polyethylene (JIS K 6748-1981) ispreferably 0.942 g/cm³ or higher.

To a high-density polyethylene or a mixture of a high-densitypolyethylene and an ultra-high molecular weight polyethylene, alow-density polyethylene may be added.

The polyolefin microporous membrane of the present invention may consistof 90% by mass or higher of polyolefin, and 10% by mass or less of otheringredients which do not have an effect on battery properties may becontained.

The polyolefin microporous membrane of the present invention has adegree of crystallinity of 60 to 85%, and a tie molecular volumefraction of from 0.7 to 1.7%.

When the degree of crystallinity and the tie molecular volume fractionare in the above range, an excellent mechanical strength and shutdownproperties can be obtained even when a polyolefin microporous membraneis complexed with a heat-resistant porous layer. Since this polyolefinmicroporous membrane has an appropriate degree of crystallinity and atie molecular volume fraction, the shape of the holes is favorablyrecovered with respect to repeated deformations generated by the volumechange of an electrode caused by charge and discharge, therebypreventing electrolyte depletion.

Here, if polyolefins are classified from the viewpoint of crystal, theyare roughly divided into: an extended-chain crystal which is formed bythe orientation of extended high polymer chain and which affects thetensile strength; a lamellar crystal which is formed by a high polymerchain which is folded in the molecule or intermolecularly; and anamorphous portion which freely moves. The amorphous portion has a tiemolecule portion which crosslinks between lamellar crystals and haveeffect on penetration strength, and a portion which is in a equilibriumstate between the crystal portion and the amorphous portion and whichcan move freely.

In the present invention, as shown in the formula (1) below, the degreeof crystallinity of polyolefins can be determined by the ratio of themelting energy measured by DSC and the theoretical melting energy of thecrystal. In the present invention, as the theoretical melting energy,289 J/g·K was employed.degree of crystallinity [%]={(measured melting energy)/(theoreticalmelting energy)}×100  (1)

In the above formula (1), the measured melting energy means the total ofthe melting energies of the extended chain and the lamellar crystal. Thehigher the degree of crystallinity, the more, the melting point, thetensile strength and the penetration strength of the polyolefinmicroporous membrane improve. That the degree of crystallinity becomeshigh means that the amorphous portion decreases.

Polymer has a portion where a part of the polymer is entangled by a tiemolecule in the amorphous portion. When the degree of crystallinitybecomes high, the amorphous portion decreases and as a result, the tiemolecule density of the amorphous portion becomes high. This amorphousportion is usually formed at the end of or on the side chain of thecrystal portion, and an entanglement at the amorphous portion restrainscrystals together. As a result, from the viewpoint of the mechanicalstrength, the entanglement leads to improvement of the penetrationstrength. However, the restraint between crystals also causes animprovement of the melting point and causes decrease in the shutdownproperties. Therefore, the degree of crystallinity is effectively in therange of 60 to 85%. A preferable range of the degree of crystallinity is60 to 80%.

As shown in the formula (2) below, the tie molecule volume fraction canbe determined by the tensile modulus of a sample to the theoreticaltensile modulus of polyolefin. The theoretical tensile modulus was 41GPa. The tensile modulus of the sample was a value obtained by dividingthe measured value by (100−porosity)/100, presuming the state that theporosity of the polyolefin is 0%.tie molecule volume fraction [%]={(1−0.01×degree ofcrystallinity)×elastic modulus of sample}/(theoretical elasticmodulus−0.01×degree of crystallinity×elastic modulus ofsample})×100  (2)

Since the tie molecule means a restraint between crystals byentanglement of the amorphous portions, the more the tie moleculesexists, the higher penetration strength is brought about. However, therestraint between the crystals causes an increase in the melting pointalong therewith, thereby causing decrease in the shutdown properties.For this reasons, in order to go together the shutdown properties andthe penetration strength, it is required to achieve a balance of the tiemolecule volume fraction. In this point of view, the tie molecule volumefraction is effectively in the range of from 0.7 to 1.7%.

The tie molecule volume fraction is preferably in the range of from 0.7to 1.5%, and preferably in the range of 1.0 to 1.5%.

In the present invention, a method of controlling the degree ofcrystallinity and the tie molecule volume fraction is not particularlylimited, and specific examples thereof include drawing conditions orheat fixation conditions of polyolefin microporous membrane, a controlof the molecular weight distribution or the branched structure of thepolyolefin used for the raw material and a control of a kneadingtemperature of a raw material.

Generally, the higher the molecular weight, the less the branchedstructures, the stronger the drawing conditions and the lower the heatfixation temperature, the better the degree of crystallinity tends toimprove. The higher the molecular weight, the more the branchedstructures and the stronger the drawing conditions, the better the tiemolecule volume fraction tends to improve.

In the present invention, particularly in order for the degree ofcrystallinity and the tie molecule volume fraction to fall within theabove ranges, a method of controlling the resin temperature at the timewhen polyolefin is melt-extruded (i.e., extruding temperature) in therange of from 190 to 220° C. is preferred.

In the polyolefin microporous membrane of the present invention, thenumber average molecular weight of polyolefin constituting themicroporous membrane is preferably from 30,000 to 80,000. When thenumber average molecular weight of the polyolefin is in theabove-mentioned range, the shutdown properties and mechanical strengthof the separator become more excellent. In this case, the tie moleculevolume fraction is preferably in the range of 1.0 to 1.7%.

The larger the number average molecular weight of polyolefin, the higherthe occurrence of entanglements of high polymer chains. When the numberaverage molecular weight is not more than 80,000, the flowability of thepolyolefin is favorable, and the shutdown properties can be maintainedfavorable. When the number average molecular weight is not less than30,000, the mechanical strength can be maintained.

Among others, the number average molecular weight of polyolefin ispreferably from 30,000 to 50,000.

When two or more polyolefins are mixed to be used, the number averagemolecular weight measured in the mixed state is denoted as the numberaverage molecular weight of the present invention.

In the polyolefin microporous membrane of the present invention, thenumber of short chain branches contained on 1000 carbon atoms on themain chain of polyolefin constituting the microporous membrane ispreferably from 1 to 5. When the number of short chain branches inpolyolefin is in the above-mentioned range, the shutdown properties andthe mechanical strength of the separator become excellent. In this case,the tie molecule volume fraction is preferably in the range of from 0.7to 1.5%, and more preferably 1.0 to 1.5%.

The larger the number of short chain branches of the polyolefin, thehigher the occurrence of entanglement of high polymer chains. When thenumber of short chain branches on 1000 carbon atoms on the main chain isnot more than 5, the flowability of the polyolefin is favorable, and theshutdown properties can be maintained favorable. When the number ofshort chain branches is not less than one, the mechanical strength canbe maintained. Among the above-mentioned, the number of short chainbranches is more preferably, from 1 to 2.

According to the method described in pp. 590-594 of “Handbook of PolymerAnalysis”, (The Japan Society for Analytical Chemistry, ResearchCommittee of Polymer Analysis), the number of short chain branches ofthe polyolefin per 1000 carbon atoms on the main chain can be determinedby using a characteristic absorption specific to a branch measured byinfrared spectrophotometer and the conversion factor described in“Handbook of Polymer Analysis”.

In the polyolefin microporous membrane, since the molecular weight ofpolyolefin, the tie molecular volume fraction and the number of shortchain branches are controlled in appropriate ranges, the shape of theholes is favorably recovered with respect to repeated deformationsgenerated by the volume change of an electrode caused by charge anddischarge, thereby preventing electrolyte depletion.

In the following, preferred physical properties of polyolefin of thepresent invention will be described.

From the viewpoint of the energy density, the load characteristics, themechanical strength and the handling properties of non-aqueous secondarybattery, the membrane thickness of a polyolefin microporous membrane ofthe present invention is preferably 5 to 25 μm.

From the viewpoint of the permeability, the mechanical strength and thehandling properties, the porosity of a polyolefin microporous membraneof the present invention is preferably 30 to 60%. Further preferably,the porosity is 40 to 60%.

From the viewpoint of obtaining the mechanical strength and the membraneresistance in good balance, the Gurley number (JIS P 8117) of apolyolefin microporous membrane of the present invention is preferably50 to 500 sec/100 cc.

From the viewpoint of the load characteristics of non-aqueous secondarybattery, the membrane resistance of a polyolefin microporous membrane ofthe present invention is preferably 0.5 to 5 ohm·cm².

The penetration strength of a polyolefin microporous membrane of thepresent invention is preferably 250 g or higher. When the penetrationstrength is 250 g or higher, in the production of a non-aqueoussecondary battery, the generation of pinholes or the like due tounevenness of electrodes, impact or the like is prevented and a shortcircuit in the non-aqueous secondary battery can be avoided.

The tensile strength of a polyolefin microporous membrane of the presentinvention is preferably 10 N or higher. When the tensile strength is 10N or higher, the breakage of a separator can be prevented when theseparator is wound in the production of a non-aqueous secondary battery.

The shutdown temperature of a polyolefin microporous membrane of thepresent invention is 130 to 150° C. The term “shutdown temperature”refers to the temperature at which the resistance value is 10³ ohm·cm².When the shutdown temperature is 130° C. or higher, the occurrence of ashutdown phenomenon at a low temperature is prevented as well as, aso-called meltdown phenomenon in which polyolefin microporous membraneis completely melt to cause a so-called short circuit phenomenon can beeffectively prevented. When the shutdown temperature is 150° C. orlower, a safety function at a high temperature can be expected. Thepreferred shutdown temperature is 135 to 145° C.

The heat shrinkage ratio of a polyolefin microporous membrane of thepresent invention at 105° C. is preferably 5 to 40%. When the heatshrinkage ratio is in this range, a separator for non-aqueous secondarybattery obtained by processing a polyolefin microporous membrane hasbalanced shape stability and shutdown properties.

[Method of Producing Polyolefin Microporous Membrane]

The production method of the polyolefin microporous membrane of thepresent invention is not particularly restricted, and preferably,specifically includes the following processes (1) to (6). Polyolefinwhich is used as the raw material is as described above.

(1) Preparation of Polyolefin Solution

A solution in which polyolefin is dissolved in a solvent is prepared (adrawing process). At this time, the solution may be prepared by mixingthe solvent. Examples of the solvent include paraffin, liquid paraffin,paraffin oil, mineral oil, castor oil, tetralin, ethylene glycol,glycerin, decaline, toluene, xylene, diethyltriamine, ethyldiamine,dimethyl sulphoxide and hexane. From the viewpoint of controlling thedegree of crystallinity, a mixed solvent containing a volatile solventand a nonvolatile solvent is preferred. Examples of the volatile solventinclude solvents having a boiling point lower than 300° C. atatmospheric pressure, such as decaline, toluene, xylene,diethyltriamine, ethyldiamine, dimethyl sulphoxide, hexane, tetralin,ethylene glycol and glycerin. Examples of the nonvolatile solventinclude solvents having a boiling point of 300° C. or higher atatmospheric pressure, such as paraffin, liquid paraffin, paraffin oil,mineral oil and castor oil. As the mixed solvent, the combination ofdecaline and paraffin is preferred.

When a mixed solvent composed of a volatile solvent and a non-volatilesolvent is used, the mixed solvent is preferably added in an amount offrom 65 to 99 parts by mass with respect to 100 parts of the totalamount of polyolefin and the mixed solvent.

The concentration of the polyolefin in the polyolefin solution ispreferably 1 to 35% by mass, and more preferably 10 to 30% by mass. Whenthe concentration of polyolefin is 1% by mass or higher, a gelcomposition obtained by cold gelation is hard to deform since the gelcomposition can be maintained so as not to highly swell by the solvent,which provides good handling properties. On the other hand, when theconcentration of polyolefin is 35% by mass or lower, the discharge ratecan be maintained since the pressure during extrusion can be restrained,which provide excellent productivity. Orientation in the extrusionprocess is less likely to proceed, which has advantage in securingdrawability or uniformity.

Here, the polyolefin contains an ultra-high molecular weightpolyethylene having a weight-averaged molecular weight of 1,000,000 orhigher and a high-density polyethylene having a density of 0.942 g/cm³.

In order to obtain the physical properties of crystal of the presentinvention, the kneading temperature in preparing a polyolefin solutionwhich is a source is preferably from 190 to 220° C. The kneadingtemperature is further preferably 195 to 208° C.

(2) Extrusion of Polyolefin Solution

The prepared solution is kneaded with a monoaxial extruder or a biaxialextruder, and extruded at a temperature from the melting point to themelting point +60° C. through a T-die or I-die (extrusion process).Preferably, a biaxial extruder is employed. Subsequently, the extrudedsolution is allowed to pass through a chill roll or a cooling bath to becooled and form a gel composition. In this case, it is preferred thatthe extruded solution be quenched to a temperature below the gelationtemperature to be gelled.

(3) Removing Solvent

Next, the volatile solvents are removed from the gel composition (thefirst solvent removal process). When a volatile solvent is used, thesolvent can also be removed from the gel composition by evaporating byheating or the like which is also served as a pre-heating process. Whena nonvolatile solvent is used, the solvent can be removed by, forexample, squeezing out by applying a pressure. There is not necessarilya need to completely remove the solvents.

(4) Drawing of Gel Composition

After removing solvents, the gel composition is drawn (drawing process).Here, prior to the drawing process, a relaxing process may be performed.In the drawing process, the gel molding is heated, and biaxially drawnat a predetermined magnification by using a normal tenter method, a rollmethod, a rolling method or a combination thereof. The biaxial drawingmay be performed simultaneously or successively. The drawing may beperformed in longitudinal multistep, or three- or four-step.

The drawing temperature is preferably from 90° C. to the melting pointof the polyolefin, and further preferably from 100 to 120° C. When theheating temperature exceeds the melting point, the gel molding melts,and therefore a drawing cannot be performed. When the heatingtemperature is less than 90° C., the gel molding softens insufficientlyand a membrane breakage during drawing tends to occur, whereby a drawingat a high magnification may be difficult.

The drawing magnification varies depending on the thickness of theoriginal fabric, and is at least two times or larger, and preferably 4to 20 times in one axis direction. In particular, from the viewpoint ofcontrolling crystal parameters, it is preferred that the drawingmagnification be 4 to 10 times in the machine direction, and 6 to 15times in the direction perpendicular to the machine direction.

After the drawing, a heat fixation is performed as required to provide aheat dimensional stability.

(5) Extraction and Removal of Solvent

The gel composition after drawing is immersed in an extraction solventto extract and remove a nonvolatile solvent (the second solvent removalprocess). Examples of extraction solvent include easily volatile solventsuch as hydrocarbons such as pentane, hexane, heptane, cyclohexane,decaline and tetralin; chlorinated hydrocarbons such as methylenechloride, carbon tetrachloride and methylene chloride;fluorohydrocarbons such as trifluoroethane; and ethers such as diethylether and dioxane. These solvents are appropriately selected dependingon the nonvolatile solvent which is used for dissolving the polyolefincomposition, and may be used alone or in combination. As for theextraction of the solvent, the solvent in the microporous membrane isremoved to less than 1% by mass.

(6) Annealing of Microporous Membrane

The microporous membrane is heat set by annealing. The annealing isperformed at from 80 to 150° C. From the viewpoint that the microporousmembrane has a predetermined heat shrinkage ratio in the presentinvention, the annealing temperature is preferably from 115 to 135° C.

[Separator for Non-Aqueous Secondary Battery]

(Separator for Non-Aqueous Secondary Battery of First Embodiment)

The separator for non-aqueous secondary battery of the first embodimentof the present invention is a separator for non-aqueous secondarybattery including the above-mentioned polyolefin microporous membraneand a heat-resistant porous layer(s) containing heat resistant resinlayered on one side or both sides of the polyolefin microporousmembrane.

By such a separator for non-aqueous secondary battery, a shutdownfunction can be obtained by the polyolefin microporous membrane, and atthe same time, since polyolefin is retained even at a temperature higherthan the shutdown temperature by the heat-resistant porous layer, ameltdown is less likely to occur, whereby safety at a high temperaturecan be secured. Therefore, by the separator for non-aqueous secondarybattery, a non-aqueous secondary battery having an excellent safely canbe obtained.

In the separator for non-aqueous secondary battery, from the viewpointof energy density of non-aqueous secondary battery, the whole membranethickness is preferably 30 μm or smaller.

From the viewpoint of the permeability, the mechanical strength and thehandling properties, the porosity of the separator for non-aqueoussecondary battery is preferably 30 to 70%. The porosity is morepreferably 40 to 60%.

From the viewpoint of improved balance between the mechanical strengthand the membrane resistance, the Gurley number (JIS P8117) of theseparator for non-aqueous secondary battery is preferably 100 to 500sec/100 cc.

From the viewpoint of the load characteristics of non-aqueous secondarybattery, the membrane resistance of the separator for non-aqueoussecondary battery is preferably 1.5 to 10 ohm·cm².

The penetration strength of the separator for non-aqueous secondarybattery is preferably 250 to 1000 g. When the penetration strength is250 g or larger, pinholes or the like due to unevenness of electrodes,impact or the like are not likely to be generated when a non-aqueoussecondary battery is produced, and the generation of a short circuit ofthe non-aqueous secondary battery can be restrained.

From the viewpoint of the resistance to the breakage when a separator iswound when producing a non-aqueous secondary battery, the tensilestrength of separator for non-aqueous secondary battery is preferably 10N or higher.

The shutdown temperature of the separator for non-aqueous secondarybattery is preferably 130 to 155° C. When the shutdown temperature is130° C. or higher, a meltdown does not occur at a low temperature, whichis highly safe. On the other hand, when the shutdown temperature is 155°C. or lower, safety at a high temperature can be expected. The shutdowntemperature is more preferably 135 to 150° C.

The heat shrinkage ratio of the separator for non-aqueous secondarybattery at 105° C. is preferably 0.5 to 10%. When the heat shrinkageratio is in this range, the separator for non-aqueous secondary batteryhas a good balance of the shape stability and the shutdown properties.The heat shrinkage ratio is more preferably 0.5 to 5%.

(Heat-Resistant Porous Layer)

In the separator for non-aqueous secondary battery, examples of theheat-resistant porous layer include layers having a porous structuresuch as microporous membrane-shaped, nonwoven fabric-shaped,paper-shaped or other three-dimensional network-shaped structure. As theheat-resistant porous layer, from the viewpoint of obtaining moreexcellent heat resistance, microporous membrane-shaped layer ispreferred. The term “microporous membrane-shaped layer” means a layerwhich has a large number of micropores inside and has a structure inwhich these micropores are connected to each other, wherein gases orliquids can pass from one side of the layer to the other side of thelayer.

The term “heat resistance” means characteristics in which melting ordecomposition does not occur in the temperature region lower than 200°C.

The heat-resistant porous layer may be on both sides or one side of thepolyolefin microporous membrane. From the viewpoint of the handlingproperties, the durability and the inhibitory effect of the heatshrinkage of the separator, the heat-resistant porous layer ispreferably on both sides of the polyolefin microporous membrane.

In order to fix a heat-resistant porous layer on a substrate, the methodin which the heat-resistant porous layer is formed directly on thesubstrate by coating is preferred. Other methods such as a method inwhich a sheet of a separately produced heat-resistant porous layer isadhered to a substrate by an adhesive or the like, or a method ofthermal fusion bonding or pressure bonding can also be employed.

When the heat-resistant porous layer is formed on both sides of thepolyolefin microporous membrane, the total of the thicknesses of theheat-resistant porous layers is preferably 3 μM to 12 μm. When theheat-resistant porous layer is formed only on one side of the polyolefinmicroporous membrane, the thickness of the heat-resistant porous layeris preferably 3 μm to 12 μm. Such a range of the membrane thickness ispreferred also from the viewpoint of the effect of preventing liquiddepletion.

From the viewpoint of the effect of preventing liquid depletion, theporosity of the heat-resistant porous layer is preferably in the rangeof 30 to 90%. The porosity is more preferably 30 to 70%.

—Heat Resistant Resin—

The heat resistant resin used in the present invention is suitably apolymer having a melting point of 200° C. or higher, or a polymer nothaving a melting point and having a decomposition temperature of 200° C.or higher. Preferred examples of such a heat resistant resin favorablyinclude at least one resin selected from the group consisting of fullyaromatic polyamides, polyimides, polyamide imides, polysulfones,polyketones, polyetherketones, polyether imides and cellulose. Inparticular, from the viewpoint of the durability, fully aromaticpolyamides are suitable, and from the viewpoint of the easiness offorming a porous layer and excellence in the oxidation and reductionresistance, polymethaphenylene isophthalamide which is a meta-type fullyaromatic polyamide is further preferred.

—Inorganic Filler—

In the present invention, the heat-resistant porous layer preferablycontains an inorganic filler. The inorganic filler is not particularlylimited and specific examples thereof suitably include metal oxides suchas alumina, titania, silica and zirconia; metal carbonates such ascalcium carbonate; metal phosphates such as calcium phosphate; and metalhydroxides such as aluminium hydroxide and magnesium hydroxide. From theviewpoint of elution of impurities and the durability, such an inorganicfiller is preferably highly crystalline.

As the inorganic filler, those which undergo an endothermic reaction at200 to 400° C. are preferred. In a non-aqueous secondary battery, anexotherm accompanied by decomposition of a positive electrode is thoughtto be the most dangerous, and the decomposition occurs at about 300° C.For this reason, when the endothermic reaction generation temperature isin the range of 200 to 400° C., the inorganic filler is effective forpreventing the endotherm of the non-aqueous secondary battery.

Examples of the inorganic filler which undergoes an endothermic reactionat 200 to 400° C. include an inorganic filler composed of metalhydroxides, borate compounds or clay minerals. Specific examples of theinorganic filler include aluminium hydroxide, magnesium hydroxide,calcium aluminate, dawsonite and zinc borate. Aluminium hydroxide,dawsonite and calcium aluminate undergo an dehydration reaction at 200to 300° C.; magnesium hydroxide and zinc borate undergo an dehydrationreaction at 300 to 400° C. Therefore, at least one of these inorganicfillers is preferably used. Among others, from the viewpoint of theeffect of improving a flame resistance, the handling properties, theantistatic effect and the effect of improving the durability of abattery, metal hydroxides are preferred, and particularly, aluminiumhydroxide or magnesium hydroxide is preferred.

The above-mentioned inorganic fillers are used alone or two or more ofthese may be used in combination. These flame resistant inorganicfillers can be used by mixing as appropriate other inorganic fillers,for example, metal oxides such as alumina, zirconia, silica, magnesiaand titania; metal nitrides; metal carbides; and metal carbonates.

In the present invention, from the viewpoint of the anti-short circuitproperties at a high temperature and the formability, the averageparticle size of the inorganic filler is preferably 0.1 μm to 2 μm.

In the present invention, from the viewpoint of the effect of improvingthe heat resistance, the permeability and the handling properties, thecontent of the inorganic filler in the heat-resistant porous layer ispreferably 50 to 95% by mass.

When the heat-resistant porous layer is microporous membrane-shaped, theinorganic filler in the heat-resistant porous layer may exist in a statewhere the inorganic filler is trapped in the heat resistant resin; andwhen the heat-resistant porous layer is a nonwoven fabric or the like,the inorganic filler in the heat-resistant porous layer may exist in theconstituent fibers or may be fixed on the surface of the nonwoven fabricor the like by a binder such as resins.

(Method of Producing Heat-Resistant Porous Layer)

In the present invention, the method of forming a heat-resistant porouslayer is not particularly restricted, and the method can include, forexample, the following processes (1) to (5).

In order to fix a heat-resistant porous layer on a polyolefinmicroporous membrane, the method in which the heat-resistant porouslayer is formed directly on the polyolefin microporous membrane bycoating is preferred. Other methods such as a method in which a sheet ofa separately produced heat-resistant porous layer is adhered to apolyolefin microporous membrane by an adhesive or the like, or a methodof thermal fusion bonding or pressure bonding can also be employed.

(1) Preparing of Slurry for Coating

A heat resistant resin is dissolved in a solvent to produce a slurry forcoating. Although the solvent may be any solvent as long as the solventdissolves the heat resistant resin and is not particularly restricted,specifically, polar solvents are preferred, and examples thereof includeN-methylpyrrolidone, dimethylacetamide, dimethylformamide and dimethylsulphoxide. In addition to these polar solvents, examples of the solventalso include a solvent which is a poor solvent to the heat resistantresin. By applying such a poor solvent, a micro phase separationstructure is induced, which facilitates making a porous structure whenthe heat-resistant porous layer is formed. As the poor solvent, alcoholsare suitable, and particularly polyhydric alcohols such as glycols aresuitable.

The concentration of the heat resistant resin in the slurry for coatingis preferably 4 to 9% by mass. As required, an inorganic filler isdispersed to form a slurry for coating. In the course of dispersing theinorganic filler in the slurry for coating, when the dispersibility ofthe inorganic filler is not favorable, a method for improving thedispersibility by the surface treatment of the inorganic filler by asilane coupling agent or the like is also applicable.

(2) Slurry Coating

A slurry is coated on at least one side of the polyolefin microporousmembrane. When heat-resistant porous layers are formed on both sides ofthe polyolefin microporous membrane, from the viewpoint of reduction ofprocesses, it is preferred that the heat-resistant porous layers arecoated on both sides of the substrate at the same time. Examples of amethod of coating the slurry for coating include a knife coater method,a gravure coater method, Meyer bar method, a die coater method, areverse roll coater method, a roll coater method, a screen printingmethod, an inkjet method and a spray method. Among these, from theviewpoint of forming the coating layer uniformly, the reverse rollcoater method is suitable. When the heat-resistant porous layers arecoated on both sides of the polyolefin microporous membrane at the sametime, for example, a method can be employed in which the polyolefinmicroporous membrane is allowed to pass between a pair of Meyer bars toapply an excess amount of slurry for coating on both sides and a precisemeasurement is performed by allowing the resultant membrane to passbetween a pair of reverse roll coater to scrape an excess amount ofslurry.

(3) Coagulation of Slurry

By processing the polyolefin microporous membrane coated with a slurryfor coating with a coagulation liquid capable of coagulating a heatresistant resin, the heat resistant resin is coagulated to form aheat-resistant porous layer.

Examples of a method of processing using a coagulation liquid include amethod in which a coagulation liquid is sprayed on the surface on whichslurry for coating is coated, and a method in which a polyolefinmicroporous membrane coated with a slurry for coating is immersed in abath containing a coagulation liquid (coagulation bath). Here, when acoagulation bath is installed, the coagulation bath is preferablyinstalled at a lower position of the coating apparatus.

The coagulation liquid is not particularly restricted as long as thecoagulation liquid can coagulate a heat resistant resin, and ispreferably water or a solvent used for slurry mixed with an appropriateamount of water. Here, the amount of water mixed is preferably 40 to 80%by mass based on the coagulation liquid. When the amount of water is 40%by mass or higher, time required for the heat resistant resin tocoagulate is not too long. A portion where coagulation is not sufficientis not generated. On the other hand, when the amount of water is 80% bymass or lower, the coagulation of the surface of the heat resistantresin layer in contact with a coagulation liquid proceeds at anappropriate speed, and the surface thereof is made sufficiently porousand the degree of crystallization is appropriate. Further, the cost ofrecovering the solvent can be kept low.

(4) Removal of Coagulation Liquid

The coagulation liquid used for the coagulation of the slurry is removedby washing with water.

(5) Dry

Water is removed by drying from a sheet of the polyolefin microporousmembrane on which a heat resistant resin coating layer is formed. Themethod of drying is not particularly restricted, and the dryingtemperature is preferably 50 to 80° C. When a high drying temperature isapplied, in order to avoid generating a dimension change due to heatshrinkage, a method of allowing to be in contact with a roll ispreferably applied.

(Separator for Non-Aqueous Secondary Battery of Second Embodiment)

The separator for non-aqueous secondary battery of the second embodimentof the present invention is a separator for non-aqueous secondarybattery including the above-mentioned polyolefin microporous membraneand an adhesive porous layer(s) containing a vinylidene fluoride resinlayered on one side or both sides of the polyolefin microporousmembrane.

By such a separator for non-aqueous secondary battery, due to layeringof the adhesive porous layers containing a vinylidene fluoride resin onone side or both sides of the polyolefin microporous membrane, theadherence between the separator and the electrodes increases. For thisreason, in addition to the mechanical strength, the shutdown propertiesand the effect of preventing liquid depletion, the adhesive porous layerexerts an excellent ion permeability and an electrolyte retention. Bythis, the cycle properties of the battery significantly improve.

(Adhesive Porous Layer)

The adhesive porous layer has a large number of micropores inside andhas a structure in which these micropores are connected to each other,wherein gases or liquids can pass from one side of the layer to theother side of the layer.

The adhesive porous layer may be on both sides or one side of thepolyolefin microporous membrane. From the viewpoint of preventing a curlof the separator and from the viewpoint that both sides of the separatoradhere to the positive and negative electrodes respectively whereby thecycle properties of the battery further improve, the adhesive porouslayer is preferably on both sides of the polyolefin microporous membranerather than only on one side of the polyolefin microporous membrane.

From the viewpoint of adhesiveness to the electrodes and increase incapacity of the battery, the membrane thickness of the adhesive porouslayer is preferably 1 μm to 10 μm per one side.

From the viewpoint of the ion permeability and the electrolyteretention, the porosity of the adhesive porous layer is preferably 60 to80%.

<Vinylidene Fluoride Resin>

The vinylidene fluoride resin contained in the adhesive porous layer ispreferably at least one of (i) and (ii) below:

(i) polyvinylidene fluoride

(ii) a copolymer composed of vinylidene fluoride and at least one ofhexafluoropropylene, chlorotrifluoroethylene, hexafluoroethylene andethylene.

In particular, as the vinylidene fluoride resin, from the viewpoint ofadhesiveness to the electrodes, a copolymer of vinylidene fluoride andhexafluoropropylene is preferred.

(Method of Producing Adhesive Porous Layer)

In the present invention, the method of forming an adhesive porous layeris not particularly restricted, and for example, the wet film formingmethod described below is employed to form an adhesive porous layer. Thewet film forming method is a film forming method in which a dopeobtained by blending and melting a vinylidene fluoride resin, an organicsolvent which dissolves the vinylidene fluoride resin and which iscompatible with water and a phase separating agent (gelation agent orboring agent) is coated on the polyolefin microporous membrane, and thenimmersed in an aqueous coagulation bath to coagulate the vinylidenefluoride resin, followed by washing with water and drying to form aporous layer. This wet film forming method is suitable because theporosity and the pore size of adhesive porous layer can be easilycontrolled by the composition of the dope and the composition of thecoagulation bath.

As the organic solvent, any solvent can be suitably used as long as thesolvent can dissolve a vinylidene fluoride resin and is compatible withwater. Specific examples of the organic solvent suitably includeN-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc),N,N-dimethylformamide (DMF), dimethyl sulphoxide (DMSO) andacetonitrile, and these may be mixed to be used. The concentration ofvinylidene fluoride resin in the dope is preferably 5 to 25% by mass.

As the phase separating agent, any agent can be used as long as theagent is a poor solvent to the vinylidene fluoride resin and compatiblewith water. Specific examples of the phase separating agent suitablyinclude water and alcohols, and particularly suitably include propyleneglycols containing polymer and polyhydric alcohols such as ethyleneglycol, tripropylene glycol (TPG), 1,3-butanediol, 1,4-butanediol,polyethylene glycol monoethyl ether, methanol, ethanol and glycerin. Theconcentration of the phase separating agent in the dope is preferably 0to 60% by mass based on the mixed solvent of the organic solvent and thephase separating agent.

As the coagulation bath, a mixed liquid of water, and the organicsolvent and the phase separating agent used for the dope is suitablyused. The percentage of water is preferably 30 to 90% by mass. Thequantitative ratio of the organic solvent and the phase separating agentpreferably corresponds to the quantitative ratio thereof in the dope atthe point of producing.

In the present invention, the adhesive porous layer can also be formedby the dry film forming method described below. The dry film formingmethod is a film forming method in which a dope in a solution stateobtained by blending and melting a vinylidene fluoride resin, a volatilesolvent which dissolves the vinylidene fluoride resin and a plasticizeris coated on a polyolefin microporous membrane, and then the volatilesolvent is removed by drying, followed by extracting the plasticizerwith a volatile solvent which dissolves a plasticizer and does notdissolve a vinylidene fluoride resin and drying to form a porous layer.

[Non-Aqueous Secondary Battery]

The non-aqueous secondary battery of the present invention includes apositive electrode, a negative electrode and a separator for non-aqueoussecondary battery having the above-mentioned configuration and beingdisposed between the positive electrode and the negative electrode, andis configured such that an electromotive force is obtained by doping anddedoping lithium. The non-aqueous secondary battery has a structure inwhich battery elements which are a negative electrode and a positiveelectrode opposing via a separator are impregnated with an electrolyteand these elements are enclosed in an outer package.

The negative electrode has a structure in which a negative electrodemixture composed of a negative-electrode active material, an auxiliaryconductive agent and a binder is formed on a collecting body. Examplesof the negative-electrode active material include a material on whichlithium can be electrochemically doped, such as carbon materials,silicon, aluminium, tin or Wood's metal. In particular, from theviewpoint of taking advantage of the effect of preventing liquiddepletion due to the separator of the present invention, as thenegative-electrode active material, those having a volume change of 3%or higher during the process of dedoping lithium are preferably used.Examples of such a negative-electrode active material include Sn, SnSb,Ag₃Sn, artificial graphite, graphite, Si, SiO and V₅O₄.

Examples of auxiliary conductive agent include carbon materials such asacetylene black and Ketjenblack. The binder is composed of an organicpolymer such as polyvinylidene fluoride or carboxymethylcellulose.Examples of the collecting body can include copper foil, stainless foiland nickel foil.

The positive electrode has a structure in which a positive electrode mixcomposed of a positive-electrode active material, an auxiliaryconductive agent and a binder is formed on a collecting body. Examplesof the positive-electrode active material include lithium-containingtransition metal oxides, such as LiCoO₂, LiNiO₂, LiMn_(0.5)Ni_(0.5)O₂,LiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂, LiMn₂O₄, LiFePO₄, LiCo_(0.5)Ni_(0.5)O₂ andLiAl_(0.25)Ni_(0.75)O₂. In particular, from the viewpoint of takingadvantage of the effect of preventing liquid depletion due to theseparator of the present invention, as the positive-electrode activematerial, those having a volume change of 1% or higher during theprocess of dedoping lithium are preferably used. Examples of such apositive-electrode active material include LiMn₂O₄, LiCoO₂, LiNiO₂,LiCo_(0.5)Ni_(0.5)O₂ and LiAl_(0.25)Ni_(0.75)O₂. Examples of auxiliaryconductive agent include carbon materials such as acetylene black andKetjenblack. The binder is composed of an organic polymer such aspolyvinylidene fluoride. Examples of the collecting body can includealuminum foil, stainless foil and titanium foil. Examples of thecollecting body can include aluminum foil, stainless foil and titaniumfoil.

The electrolyte has a constitution in which a lithium salt is dissolvedin a non-aqueous solvent. Examples of the lithium salt include LiPF₆,LiBF₄ and LiClO₄. Examples of the non-aqueous solvent includepropylenecarbonate, ethylene carbonate, dimethylcarbonate,diethylcarbonate, ethylmethylcarbonate, γ-butyrolactone and vinylenecarbonate. These may be used alone or mixed to be used.

Examples of the outer package include a metal can or aluminum laminatedpackaging. Examples of the shape of battery include square shape,cylinder shape and coin shape, and the separator of the presentinvention can suitably apply any of these shapes.

EXAMPLES

In the following, the present invention will now be described morespecifically by way of Examples, but not limited thereto, withoutdeviating from the spirit of the invention.

[Measuring Method]

The each value in the Examples was determined according to the followingmethods.

(1) Membrane Thickness

The membrane thicknesses of the polyolefin microporous membrane and theseparator for non-aqueous secondary battery were calculated by measuringthe thicknesses at 20 points by a contact type film thickness meter(manufactured by Mitutoyo Corporation) and averaging the measuredvalues. Here, a contact probe having a cylindrical shape and a diameterof the bottom surface of 0.5 cm was used.

(2) Unit Weight

The unit weights, which are weight per 1 m², of the polyolefinmicroporous membrane and the separator for non-aqueous secondary batterywere determined by measuring the weight of a sample cut out in 10 cm×10cm and dividing the measured mass by the area.

(3) Porosity

The porosities of the polyolefin microporous membrane and the separatorfor non-aqueous secondary battery were calculated by the followingformula:ε={1−Ws/(ds·t)}×100

Here, ε: porosity (%), Ws: unit weight (g/m²), ds: real density (g/cm³),and t: membrane thickness (m).

(4) Gurley Number (Air Permeability)

The Gurley numbers of the polyolefin microporous membrane and theseparator for non-aqueous secondary battery were determined based on JISP 8117.

(5) Membrane Resistance

The membrane resistances of the polyolefin microporous membrane and theseparator for non-aqueous secondary battery were determined by thefollowing method.

A sample having a size of 2.6 cm×2.0 cm is cut out. The cut out sampleis immersed in a 3% by mass methanol solution (methanol: manufactured byWako Pure Chemical Industries, Ltd.) in which nonionic surfactant(Emulgen 210P manufactured by Kao Corporation) is dissolved, and airdried. Aluminum foil having a thickness of 20 μm is cut out in 2.0cm×1.4 cm and a lead tab is attached thereto. Two sheets of suchaluminum foils are prepared and the cut out sample is sandwiched betweenthe aluminum foils so as not to short the aluminum foils. The sample isimpregnated with 1M LiBF₄-propylenecarbonate/ethylene carbonate (massratio: 1/1) which is an electrolyte. The resultant was enclosed with areduced pressure in an aluminum laminated packaging such that the tab isoutside the aluminum packaging. Such cells are produced such that one,two or three sheets of separators are in the aluminum foils. The cell isplaced in a thermostat bath at 20° C., and the resistance of the cell ismeasured by an alternating current impedance method with an amplitude of10 mV and a frequency of 100 kHz. The measured resistance value of thecell is plotted against the number of separators, and the plots arelinearly approximated to obtain the inclination. This inclination wasmultiplied by the electrode area, 2.0 cm×1.4 cm to obtain the membraneresistance per one sheet of separator (ohm·cm²).

(6) Penetration Strength

The penetration strengths of the polyolefin microporous membrane and theseparator for non-aqueous secondary battery were determined byperforming a penetration test by using a KES-G5 handy compression testermanufactured by Kato tech Co., Ltd. with the radius of curvature of theend of the needle of 0.5 mm and a penetration speed of 2 mm/sec, andmeasuring the maximum penetration load. Here, the samples were fixed ona metallic flask having a hole of φ 11.3 mm (sample holder) with apacking made of silicon rubber sandwiched.

(7) Tie Molecule Volume Fraction•Tensile Strength

The tensile strengths and the tensile modulus of the polyolefinmicroporous membrane and the separator for non-aqueous secondary batterywere measured with a load-cell load of 5 kgf and a distance betweenchucks of 50 mm, using samples cut out in 10×100 mm and a tensile tester(RTC-1225A manufactured by A&D Company, Limited). A tie molecule volumefraction was calculated from the tensile modulus obtained here by usingthe following formula. The tensile modulus of the sample was a valueobtained by dividing the measured value by (100−porosity)/100, presumingthe state that the porosity of the polyolefin is 0%.tie molecule volume fraction={(1−0.01×degree of crystallinity)×elasticmodulus of sample}/(theoretical elastic modulus−0.01×degree ofcrystallinity×elastic modulus of sample})×100

(8) Degree of Crystallinity

The polyolefin microporous membrane was cut out with a weight of 5±1 mgand the melting energy thereof was measured by using DSC (TA-2920manufactured by TA Instruments Japan Inc.). DSC was performed, settingthe rate of temperature increase at 2° C./min.

As in the formula below, the degree of crystallinity of polyolefin wasobtained by the ratio of the melting energy measured by DSC and thetheoretical melting energy of the crystal. As the theoretical meltingenergy, 289 J/g·K was employed.degree of crystallinity (%)=(measured melting energy)/(theoreticalmelting energy)×100

(9) Number Average Molecular Weight

The number average molecular weight of the polyolefin used for producinga polyolefin microporous membrane was measured by GPC (ALC/GPC 150-Cplus type, manufactured by Waters). When two or more polyolefins aremixed to be used, the number average molecular weight of the mixtureobtained by the mixing was measured.

(10) Number of Short Chain Branches

The number of short chain branches of a polyolefin microporous membranewas measured by an infrared spectrometer (Magna-750 manufactured byNicolet Instruments Corporation).

(11) Shutdown Temperature (SD Temperature)

The shutdown temperatures of the polyolefin microporous membrane and theseparator for non-aqueous secondary battery were obtained by thefollowing method.

First, a sample was punched out in a diameter of 19 mm and the punchedout sample was immersed in a methanol solution (methanol: manufacturedby Wako Pure Chemical Industries, Ltd.) in which 3% by mass of nonionicsurfactant (Emulgen 210P, manufactured by Kao Corporation) wasdissolved, and air dried. This sample was sandwiched with SUS plateshaving a diameter of 15.5 mm and the sample was impregnated with 1MLiBF₄ propylenecarbonate/ethylene carbonate (mass ratio: 1/1) (KISHIDACHEMICAL Co., Ltd.) which is an electrolyte. The resultant was enclosedin a 2032 type coin cell. Lead wires were connected to the coin cell anda thermocouple was attached thereto and the coin cell was placed in anoven. The temperature inside the coin cell was increased at a rate oftemperature increase of 1.6° C./min. and at the same time, theresistance of the cell was measured by an alternating current impedancemethod with an amplitude of 10 mV and a frequency of 100 kHz. The timewhen the resistance value was 10³ ohm·cm² or higher was regard as ashutdown, and the temperature at this time was defined as a shutdowntemperature.

(12) Heat Resistance

The heat resistance of the separator for non-aqueous secondary batterywas evaluated depending on whether or not the resistance value wasmaintained at 10³ ohm·cm² or higher until the temperature of the cellbecame 200° C. from the occurrence of a shutdown when the shutdowntemperature in the above (11) was measured. When the resistance valuewas maintained at 10³ ohm·cm² or higher, the resistance value was judgedas good (∘); and when the resistance value became lower than 10³ohm·cm², the resistance value was judged as not good (x).

(13) Heat Shrinkage Ratio

The heat shrinkage ratios of the polyolefin microporous membrane and theseparator for non-aqueous secondary battery were measured by heating thesample at 105° C. for 1 hour. The measurement direction is in themachine direction.

(14) Recovery Rate after Pressurization

By measuring the recovery rate after pressurization, the effect ofpreventing liquid depletion of the polyolefin microporous membrane andthe separator for non-aqueous secondary battery were evaluated.

First, a sample was cut out in a size of 2.6 cm×2.0 cm. The cut outsample was immersed in a methanol solution in which 3% by mass ofnonionic surfactant (Emulgen 210P manufactured by Kao Corporation) wasdissolved, and air dried. Aluminum foil having a thickness of 20 μM wascut out in 2.0 cm×1.4 cm and a lead tab was attached thereto. Two sheetsof such aluminum foils were prepared and the cut out separator wassandwiched between the aluminum foils so as not to short the aluminumfoils. For the electrolyte, an electrolyte in which 1M LiBF₄ wasdissolved in a solvent in which propylene carbonate and ethylenecarbonate are mixed at a mass ratio of 1/1 was used, and the sample wasimpregnated with this electrolyte. The resultant was enclosed with areduced pressure in an aluminum laminated packaging such that the tabwas outside the aluminum packaging. The resistance of the cell wasmeasured by an alternating current impedance method, with an amplitudeof 10 mV and a frequency of 100 kHz to obtain the resistance value (A)(ohm·cm²) before pressurization. The cell was then pressurized by aplate press at 40 MPa for 5 minutes, followed by releasing the pressure.This procedure was repeated 5 times and the resistance value (B)(ohm·cm²) of the cell in which a pressure was released after thepressurization was measured: The recovery rate after pressurization wasthen calculated by the formula below. The higher the recovery rate afterpressurization, the more excellent the effect of preventing liquiddepletion.recovery rate after pressurization (%)=resistance value (B)/resistancevalue (A)×100

(15) Weight-Averaged Molecular Weight of Polyolefin

The molecular weight of the polyolefin was measured by gel permeationchromatography (GPC) below.

To 15 mg of a sample, 20 ml of a mobile phase for GPC measurement wasadded to dissolve the sample completely at 145° C., and then theresultant was filtrated through a stainless sintered filter (pore size:1.0 μm). 400 μl of the filtrate was injected into the apparatus to besubjected to a measurement, and the weight-averaged molecular weight ofthe sample was determined.

Apparatus: Gel Permeation Chromatograph Alliance GPC2000 (manufacturedby Waters)

Column: TSKgel GMH6-HT×2+TSKgel GMH6-HT×2, manufactured by TosohCorporation

Column temperature: 140° C.

Mobile phase: o-dichlorobenzene

Detector: Differential refractive index detector (RI)

Molecular weight calibration: monodispersed polystyrene (manufactured byTosoh Corporation)

Example 1

As a polyethylene powder, GUR2126 (weight-averaged molecular weight:4,150,000, number average molecular weight: 800,000, melting point: 141°C.; manufactured by Ticona corporation) which is ultra-high molecularweight polyethylene and GURX143 (weight-averaged molecular weight:560,000, number average molecular weight: 50,000, melting point: 135°C.; manufactured by Ticona corporation) which is high-densitypolyethylene were used. A polyethylene solution was produced by makingGUR2126 and GURX143 20:80 (mass ratio) and dissolving them in a mixedsolvent of liquid paraffin (Smoil P-350, manufactured by Matsumura OilResearch Corp; boiling point: 480° C.) and decalin (manufactured by WakoPure Chemical Industries, Ltd.; boiling point: 193° C.). The compositionof the polyethylene solution is as follows: polyethylene:liquidparaffin:decalin=30:67.5:2.5 (mass ratio). Here, the polyethylenesolution was kneaded at 197° C.

This polyethylene solution was extruded from a die at 148° C. and cooledin a water bath to produce a gel tape (base tape).

The obtained base tape was dried at 60° C. for 8 minutes and 95° C. for15 minutes, and this base tape was drawn by biaxial drawing in whichlongitudinal drawing and transverse drawing were sequentially performed.Here, the longitudinal drawing was performed at a drawing ratio of 6times at a drawing temperature of 90° C., and the transverse drawing wasperformed at a drawing ratio of 9 times at a drawing temperature of 105°C. After the transverse drawing, a heat fixation was performed at 125°C. Next, the resultant was immersed in a methylene chloride bath toextract liquid paraffin and decalin. Subsequently, the resultant wasdried at 50° C. and subjected to an annealing process to obtain apolyolefin microporous membrane. The obtained polyolefin microporousmembrane had a structure in which fibril polyolefin was interlaced in anet-like shape, and which constitutes micropores.

The measured results of properties of the obtained polyolefinmicroporous membrane (membrane thickness, unit weight, porosity, Gurleynumber, membrane resistance, penetration strength, tensile strength, avariety of physical properties of crystal, shutdown (SD) temperature,heat shrinkage ratio, recovery rate after pressurization) are shown inTable 1 below. The results of polyolefin microporous membranes inExamples and Comparative Examples are also shown in Tables 1 or 2.

Example 2

A polyolefin microporous membrane was obtained in the same manner as inExample 1 except that the polyethylene solution was kneaded at 208° C.and the heat fixation temperature was 130° C.

Example 3

A polyolefin microporous membrane was obtained in the same manner as inExample 1 except that the polyethylene solution was kneaded at 201° C.and the heat fixation temperature was 120° C.

Example 4

A polyolefin microporous membrane was obtained in the same manner as inExample 1 except that GUR2126:GURX143 was adjusted to 30:70 (massratio), the polyethylene solution was kneaded at 195° C. and the heatfixation temperature was 132° C.

Example 5

A polyolefin microporous membrane was obtained in the same manner as inExample 4 except that the ratio of GUR2126 and GURX143 was adjusted to40:60 (mass ratio), and the polyethylene solution was kneaded at 205° C.

Example 6

A polyolefin microporous membrane was obtained in the same manner as inExample 4 except that the ratio of GUR2126 and GURX143 was adjusted to20:80 (mass ratio), and the polyethylene solution was kneaded at 205° C.

Example 7

A polyolefin microporous membrane was obtained in the same manner as inExample 1 except that SK-PE-20L (melting point: 106° C., manufactured bySeishin Enterprise Co., Ltd.) which is a polyethylene having a lowdensity was further used as polyethylene powder; GUR2126: GURX143:SK-PE-20L was adjusted to 30:60:10 (mass ratio); the polyethylenesolution was kneaded at 199° C.; and the heat fixation temperature was123° C.

Example 8

A polyolefin microporous membrane was obtained in the same manner as inExample 7 except that GUR2126: GURX143: SK-PE-20L was adjusted to30:70:0 (mass ratio); the polyethylene solution was kneaded at 202° C.;and the heat fixation temperature was 122° C.

Example 9

A polyolefin microporous membrane was obtained in the same manner as inExample 7 except that GUR2126: GURX143: SK-PE-20L was adjusted to30:10:60 (mass ratio); the polyethylene solution was kneaded at 200° C.;and the heat fixation temperature was 124° C.

Comparative Example 1

A polyolefin microporous membrane was obtained in the same manner as inExample 1 except that the polyethylene solution was kneaded at 181° C.and the heat fixation temperature was 110° C.

Comparative Example 2

A polyolefin microporous membrane was obtained in the same manner as inExample 1 except that the polyethylene solution was kneaded at 231° C.and the heat fixation temperature was 138° C.

Comparative Example 3

A polyolefin microporous membrane was obtained in the same manner as inExample 4 except that the heat fixation temperature was 138° C. and thepolyethylene solution was kneaded at 180° C.

Comparative Example 4

A polyolefin microporous membrane was obtained in the same manner as inExample 4 except that the ratio of GUR2126 and GURX143 was adjusted to70:30 (mass ratio), and the polyethylene solution was kneaded at 230° C.

Comparative Example 5

A polyolefin microporous membrane was obtained in the same manner as inExample 7 except that GUR2126: GURX143: SK-PE-20L was adjusted to25:75:0 (mass ratio); the polyethylene solution was kneaded at 185° C.;and the heat fixation temperature was 135° C.

Comparative Example 6

A polyolefin microporous membrane was obtained in the same manner as inExample 7 except that GUR2126: GURX143: SK-PE-20L was adjusted to30:0:70 (mass ratio); and the polyethylene solution was kneaded at 228°C.

Comparative Example 7

A polyolefin microporous membrane was obtained in the same manner as inExample 1 except that the mixing ratio of GUR2126 and GURX143 was made10:90 (mass ratio); the composition of the polyethylene solution was asfollows: polyethylene:liquid paraffin:decalin=30:45:25 (mass ratio); thepolyethylene solution was kneaded at 180° C.; the longitudinal drawingratio was set to 5.5 times; the transverse drawing ratio was set to 11times; and after the transverse drawing a heat fixation was performed at125° C.

Comparative Example 8

A polyolefin microporous membrane was obtained in the same manner as inComparative Example 7 except that the mixing ratio of GUR2126 andGURX143 was made 30:70 (mass ratio); the concentration of polyethylenewas set to 25% by mass; the composition of the polyethylene solution wasas follows: polyethylene:liquid paraffin:decalin=25:37.5:37.5 (massratio).

Comparative Example 9

A polyolefin microporous membrane was obtained in the same manner as inComparative Example 7 except that the mixing ratio of GUR2126 andGURX143 was made 50:50 (mass ratio); the concentration of polyethylenewas set to 21% by mass; the composition of the polyethylene solution wasas follows: polyethylene:liquid paraffin:decalin=21:31.5:47.5 (massratio).

Comparative Example 10

A polyolefin microporous membrane was obtained in the same manner as inComparative Example 7 except that the mixing ratio of GUR2126 andGURX143 was made 70:30 (mass ratio); the concentration of polyethylenewas set to 17% by mass; the composition of the polyethylene solution wasas follows: polyethylene:liquid paraffin:decalin=17:51:32 (mass ratio).

Comparative Example 11

A polyolefin microporous membrane was obtained in the same manner as inComparative Example 7 except that the mixing ratio of GUR2126 andGURX143 was made 30:70 (mass ratio); the concentration of polyethylenewas set to 25% by mass; the composition of the polyethylene solution wasas follows: polyethylene:liquid paraffin:decalin=21:31.5:47.5 (massratio).

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Example 8 Example 9 Film thickness [μm] 12 12 12 12 12 12 1111 11 Unit weight [g/m²] 6.3 6.4 6.3 6.2 6.0 6.2 5.5 5.6 5.6 Porosity[%] 42 41 42 43 44 43 44 43 43 Degree of [%] 80 85 60 72 79 62 72 82 60crystallinity Tie molecule [%] 1.4 1.5 0.7 1.4 1.5 0.7 1.2 1.5 1 volumefraction Number average ×10³[—] 30 30 30 50 80 30 50 50 50 molecularweight the number of short [—] 1 1 1 1 1 1 2 1 5 chain branches Gurleynumber [sec/100 cc] 176 172 176 181 185 181 169 166 166 Membrane [ohm ·cm²] 1.5 1.4 1.5 1.6 1.6 1.6 1.4 1.4 1.3 resistance Penetration [g] 343351 343 335 327 335 300 307 307 strength Tensile strength [N] 22 23 2222 23 22 21 22 22 SD temperature [° C.] 142 140 141 139 138 139 140 139140 Heat shrinkage [%] 27 26 25 31 30 32 33 32 34 ratio Recovery rateafter [%] 85 83 79 85 84 83 85 82 84 pressurization

TABLE 2 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp.Comp. Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam-ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 ple 8 ple 9 ple 10 ple 11 Filmthickness [μm] 12 12 12 12 11 11 11.5 11.5 12 14.7 10.9 Unit weight[g/m²] 6.0 6.4 6.0 6.3 5.5 5.6 6.9 6.3 7.1 7.3 6.0 Porosity [%] 44 41 4442 44 43 36 36 37 47 41 Degree of [%] 75 70 70 64 86 59 86 87 64 63 86crystallinity Tie molecule [%] 1.8 0.5 1.8 0.6 0.8 1.8 0.6 1.1 2.1 2.21.2 volume fraction Number average ×10³[—] 30 30 50 180 40 50 20 50 115180 50 molecular weight the number of short [—] 1 1 1 1 0.8 6 1 1 1 1 1chain branches Gurley number [sec/100 cc] 185 172 185 176 169 166 301306 351 199 250 Membrane [ohm · cm²] 1.6 1.4 1.6 1.5 1.6 1.4 2.641 2.5012.58 1.960 2.190 resistance Penetration [g] 327 351 327 343 300 307 380393 475 511 344 strength Tensile strength [N] 19 23 19 23 19 23 21 25 2426 22 SD temperature [° C.] 143 148 147 148 145 148 143 144 144 146 144Heat shrinkage [%] 33 27 27 30 28 33 30 31 27 28 28 ratio Recovery rateafter [%] 40 42 43 4 43 46 55 60 68 65 69 pressurization

Example 10

By using the polyolefin microporous membrane obtained in Example 1, aheat-resistant porous layer composed of a heat resistant resin and aninorganic filler was layered thereon to produce a separator fornon-aqueous secondary battery of the present invention.

Specifically, as the heat resistant resin, polymethaphenyleneisophthalamide (manufactured by TEIJIN TECHNO PRODUCTS LIMITED; CONEX)was employed. This heat resistant resin was dissolved in a mixed solventof dimethylacetamide (DMAc) and tripropylene glycol (TPG) having a massratio of 50:50. In this polymer solution, magnesium hydroxide(manufactured by Kyowa Chemical Industry Co., Ltd., KISUMA-5P, averageparticle size: 1.0 μm) as the inorganic filler was dispersed to producea slurry for coating. The concentration of polymethaphenyleneisophthalamide in the slurry for coating was adjusted to 5.5% by massand the mass ratio of polymethaphenylene isophthalamide and theinorganic filler was adjusted to 25:75. Two Meyer bars were faced toeach other and a proper amount of the coating liquid was placed betweenthem. Thereafter, a polyolefin microporous membrane was allowed to passbetween the Meyer bars on which the coating liquid were placed, and theslurry for coating was coated on the both sides of the polyolefinmicroporous membrane. Here, the clearance between the Meyer bars wereset to 20 μm and as both of two Meyer bars, #6 were employed. Thismembrane was immersed in a coagulation liquid in the mass ratio ofwater:DMAc:TPG=50:25:25 at 40° C., and then washed with water and dried.By this, a separator for non-aqueous secondary battery in whichheat-resistant porous layers were formed on both sides of the polyolefinmicroporous membrane was obtained.

The measured results of properties of the obtained separator fornon-aqueous secondary battery (membrane thickness, unit weight,porosity, Gurley number, membrane resistance, penetration strength,tensile strength, shutdown temperature, heat resistance, heat shrinkageratio, recovery rate after pressurization) are shown in Table 3. Theresults of the separator for non-aqueous secondary battery in Examplesand Comparative Examples are also shown in Tables 3 to 6.

Example 11

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Example 2 was employed.

Example 12

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Example 3 was employed.

Example 13

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that the clearance between Meyer bars wasset to 7 μm.

Example 14

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that the mixing ratio of DMAc and TPG wasmade 40:60 (mass ratio); the clearance between Meyer bars was set to 60μm; and the composition of the coagulation liquid was adjusted such thatwater:DMAc:TPG=50:30:20.

Example 15

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that the mixing ratio of DMAc and TPG wasmade 40:60 (mass ratio); the clearance between Meyer bars was set to 75μm; and the composition of the coagulation liquid was adjusted such thatwater:DMAc:TPG=50:30:20.

Example 16

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that the mixing ratio of DMAc and TPG wasmade 35:65 (mass ratio); the clearance between Meyer bars was set to 60μm; and the composition of the coagulation liquid was adjusted such thatwater:DMAc:TPG=50:32:18.

Example 17

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that the mixing ratio of DMAc and TPG wasmade 70:30 (mass ratio); and the composition of the coagulation liquidwas adjusted such that water:DMAc:TPG=50:15:35.

Example 18

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Example 4 was employed.

Example 19

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Example 5 was employed.

Example 20

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Example 6 was employed.

Example 21

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 18 except that the clearance between Meyer bars wasset to 7 μm.

Example 22

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 18 except that the mixing ratio of DMAc and TPG wasmade 40:60 (mass ratio); the clearance between Meyer bars was set to 60μm; and the composition of the coagulation liquid was adjusted such thatwater:DMAc:TPG=50:30:20.

Example 23

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 18 except that the mixing ratio of DMAc and TPG wasmade 40:60 (mass ratio); the clearance between Meyer bars was set to 75μm; and the composition of the coagulation liquid was adjusted such thatwater:DMAc:TPG=50:30:20.

Example 24

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 18 except that the mixing ratio of DMAc and TPG wasmade 35:65 (mass ratio); the clearance between Meyer bars was set to 60μm; and the composition of the coagulation liquid was adjusted such thatwater:DMAc:TPG=50:32:18.

Example 25

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 18 except that the mixing ratio of DMAc and TPG wasmade 70:30 (mass ratio); and the composition of the coagulation liquidwas adjusted such that water:DMAc:TPG=50:15:35.

Example 26

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Example 7 was employed.

Example 27

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Example 8 was employed.

Example 28

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Example 9 was employed.

Example 29

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 26 except that the clearance between Meyer bars wasset to 7 μm.

Example 30

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 26 except that the mixing ratio of DMAc and TPG wasmade 40:60 (mass ratio); the clearance between Meyer bars was set to 60μm; and the composition of the coagulation liquid was adjusted such thatwater:DMAc:TPG=50:30:20.

Example 31

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 26 except that the mixing ratio of DMAc and TPG wasmade 40:60 (mass ratio); the clearance between Meyer bars was set to 75μm; and the composition of the coagulation liquid was adjusted such thatwater:DMAc:TPG=50:30:20.

Example 32

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 26 except that the mixing ratio of DMAc and TPG wasmade 35:65 (mass ratio); the clearance between Meyer bars was set to 60μm; and the composition of the coagulation liquid was adjusted such thatwater:DMAc:TPG=50:32:18.

Example 33

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 26 except that the mixing ratio of DMAc and TPG wasmade 70:30 (mass ratio); and the composition of the coagulation liquidwas adjusted such that water:DMAc:TPG=50:15:35.

Comparative Example 12

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Comparative Example 1 was employed.

Comparative Example 13

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Comparative Example 2 was employed.

Comparative Example 14

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Comparative Example 3 was employed.

Comparative Example 15

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Comparative Example 4 was employed.

Comparative Example 16

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Comparative Example 5 was employed.

Comparative Example 17

A separator for non-aqueous secondary battery was obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembrane, the one produced in Comparative Example 6 was employed.

Comparative Examples 18 to 22

Separators for non-aqueous secondary battery were obtained in the samemanner as in Example 10 except that, as the polyolefin microporousmembranes, those produced in Comparative Examples 7 to 11 were employedindividually.

TABLE 3 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 10 ple 11ple 12 ple 13 ple 14 ple 15 ple 16 ple 17 Polyolefin microporousmembrane used Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple2 ple 3 ple 1 ple 1 ple 1 ple 1 ple 1 Heat-resistant Film thickness [μm]6 6 6 2 20 25 20 6 porous layer Unit weight [g/m²] 5.1 5.1 5.1 2.7 12.213.2 8.4 8.9 Porosity [%] 60 60 60 35 71 75 80 30 Complex Film thickness[μm] 18 18 18 14 32 37 32 18 membrane Unit weight [g/m²] 11.3 11.4 11.38.8 18.3 19.2 14.5 14.9 Porosity [%] 48 47 48 41 60 64 66 38 Gurleynumber [sec/100 cc] 296 292 296 280 310 315 320 310 Membrane resistance[ohm · cm²] 2.6 2.5 2.6 2.3 3 3.2 3.2 2.9 Penetration strength [g] 350358 350 345 360 360 360 352 Tensile strength [N] 22 23 22 22 23 23 23 22Shutdown [° C.] 143 141 142 143 143 143 143 143 temperature Heatresistance [—] ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ Heat shrinkage ratio [%] 12 11 10 14 10 910 11 Recovery rate after [%] 99 97 93 82 89 75 79 85 pressurization

TABLE 4 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 18 ple 19ple 20 ple 21 ple 22 ple 23 ple 24 ple 25 Polyolefin microporousmembrane used Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 4 ple5 ple 6 ple 4 ple 4 ple 4 ple 4 ple 4 Heat-resistant Film thickness [μm]6 6 6 2 20 25 20 6 porous layer Unit weight [g/m²] 5.1 5.1 5.1 2.7 12.213.2 8.4 8.9 Porosity [%] 60 60 60 35 71 75 80 30 Complex Film thickness[μm] 18 18 18 14 32 37 32 18 membrane Unit weight [g/m²] 11.2 11.1 11.28.8 18.3 19.2 14.5 14.9 Porosity [%] 49 49 49 42 61 65 66 39 Gurleynumber [sec/100 cc] 301 305 301 290 311 321 315 307 Membrane resistance[ohm · cm²] 2.7 2.7 2.7 2.4 3 3.1 3.1 2.9 Penetration strength [g] 342334 342 340 350 355 352 345 Tensile strength [N] 22 23 22 22 23 23 23 22Shutdown [° C.] 140 139 140 140 140 140 140 140 temperature Heatresistance [—] ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ Heat shrinkage ratio [%] 16 15 17 18 1412 12 15 Recovery rate after [%] 99 98 97 82 89 75 79 85 pressurization

TABLE 5 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 26 ple 27ple 28 ple 29 ple 30 ple 31 ple 32 ple 33 Polyolefin microporousmembrane used Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 7 ple8 ple 9 ple 7 ple 7 ple 7 ple 7 ple 7 Heat-resistant Film thickness [μm]6 6 6 2 20 25 20 6 porous layer Unit weight [g/m²] 5.1 5.1 5.1 2.7 12.213.2 8.4 8.9 Porosity [%] 60 60 60 35 71 75 80 30 Complex Film thickness[μm] 17 17 17 14 32 37 32 18 membrane Unit weight [g/m²] 10.6 10.7 10.78.8 18.3 19.2 14.5 14.9 Porosity [%] 50 49 49 40 60 64 65 37 Gurleynumber [sec/100 cc] 289 286 286 275 310 315 319 304 Membrane resistance[ohm · cm²] 2.5 2.5 2.4 2.3 2.6 2.8 2.8 2.6 Penetration strength [g] 306313 313 301 310 315 315 310 Tensile strength [N] 21 22 22 21 22 22 22 21Shutdown [° C.] 141 140 141 141 141 141 141 141 temperature Heatresistance [—] ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ Heat shrinkage ratio [%] 18 17 19 19 1615 15 17 Recovery rate after [%] 99 96 98 82 89 75 79 85 pressurization

TABLE 6 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp.Comp. Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam-ple 12 ple 13 ple 14 ple 15 ple 16 ple 17 ple 18 ple 19 ple 20 ple 21ple 22 Polyolefin microporous membrane used Comp. Comp. Comp. Comp.Comp. Comp. Comp. Comp. Comp. Comp. Comp. Exam- Exam- Exam- Exam- Exam-Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 6ple 7 ple 8 ple 9 ple 10 ple 11 Heat- Film [μm] 6 6 6 6 6 6 6 6 6 6 6resistant thickness porous layer Unit weight [g/m²] 5.1 5.1 5.1 5.1 5.15.1 5.1 5.1 5.1 5.1 5.1 Porosity [%] 60 60 60 60 60 60 60 60 60 60 60Complex Film [μm] 18 18 18 18 17 17 17.5 16.5 18 20.7 16.9 membranethickness Unit weight [g/m²] 11.1 11.4 11.1 11.3 10.6 10.7 12.0 11.412.2 12.4 11.1 Porosity [%] 49 47 49 48 50 49 44 45 45 51 48 Gurley[sec/100 cc] 305 292 305 296 289 286 381 393 445 303 380 number Membrane[ohm · cm²] 2.7 2.5 2.7 2.6 2.7 2.5 3.7 3.6 3.7 3.1 3.3 resistancePenetration [g] 334 358 334 350 306 313 391 406 487 526 356 strengthTensile [N] 19 23 19 23 19 23 22 26 25 27 23 strength Shutdown [° C.]144 149 148 149 146 149 144 145 145 147 145 temperature Heat [—] ∘ ∘ ∘ ∘∘ ∘ ∘ ∘ ∘ ∘ ∘ resistance Heat [%] 18 12 12 15 13 18 14 15 19 19 14shrinkage ratio Recovery rate [%] 63 58 61 65 70 75 66 79 73 67 78 afterpressurization

Examples 34 to 66, Comparative Examples 23 to 44

Non-aqueous secondary batteries were produced by using the polyolefinmicroporous membranes or the separators for non-aqueous secondarybattery produced in Examples 1 to 33 and Comparative Examples 1 to 22,and the cycle properties thereof were evaluated.

—Preparation of Test Batteries—

˜Preparation of Positive Electrode˜

By using N-methyl-pyrrolidone, 89.5 parts by mass of lithium cobaltate(LiCoO₂: manufactured by NIPPON CHEMICAL INDUSTRIAL CO., LTD.), 4.5parts by mass of acetylene black (DENKA BLACK manufactured by DENKIKAGAKU KOGYOU KABUSHIKI KAISHA) and 6 parts by mass of polyvinylidenefluoride (manufactured by KUREHA CORPORATION) were kneaded to produce aslurry. The obtained slurry was coated on an aluminum foil having athickness of 20 μm, dried, and then pressed to obtain a 100 μm positiveelectrode.

˜Preparation of Negative Electrode˜

By using N-methyl-2pyrrolidone, 87 parts by mass of meso phase carbonmicrobeads (MCMB: manufactured by Osaka Gas Chemicals Co., Ltd.), 3parts by mass of acetylene black (trade name: DENKA BLACK, manufacturedby DENKI KAGAKU KOGYOU KABUSHIKI KAISHA) and 10 parts by mass ofpolyvinylidene fluoride (manufactured by KUREHA CORPORATION) werekneaded to produce a slurry. The obtained slurry was coated on a copperfoil having a thickness of 18 μm, dried, and then pressed to obtain a 90μm negative electrode.

˜Preparation of Battery˜

The polyolefin microporous membrane or separator for non-aqueoussecondary battery produced in Examples 1 to 33 and Comparative Examples1 to 22 was sandwiched between the positive electrode and the negativeelectrode respectively. The resultant was impregnated with anelectrolyte and enclosed in an outer package composed of an aluminumlaminate film to produce non-aqueous secondary batteries in Examples 12to 22 and Comparative Examples 5 to 8. Here, as the electrolyte, 1MLiPF₆ ethylene carbonate/ethylmethylcarbonate (mass ratio: 3/7)(manufactured by KISHIDA CHEMICAL Co., Ltd.) was employed.

This test battery had a positive electrode area of 2×1.4 cm², a negativeelectrode area of 2.2×1.6 cm² and a set capacity of 8 mAh (in the rangeof 4.2 V-2.75 V).

—Evaluation of Cycle Properties—

For each of the obtained non-aqueous secondary batteries, 4.0 V ofconstant-current and constant-voltage charge and 2.75 V ofconstant-current discharge were repeated for 100 cycles and then thedischarged capacity was measured. The value obtained by dividing thedischarged capacity after 100 cycles by the discharged capacity after 3cycles was defined as a discharged capacity retention (%), which wasemployed as an index of cycle properties. The measured results are shownin Tables 7 to 8.

TABLE 7 Cycle properties (discharged Battery Separator used capacityretention [%]) Example 34 Example 1 96 Example 35 Example 2 91 Example36 Example 3 92 Example 37 Example 10 77 Example 38 Example 11 63Example 39 Example 12 95 Example 40 Example 13 90 Example 41 Example 1491 Example 42 Example 15 86 Example 43 Example 16 85 Example 44 Example17 81 Example 45 Example 4 98 Example 46 Example 5 95 Example 47 Example6 93 Example 48 Example 18 75 Example 49 Example 19 60 Example 50Example 20 97 Example 51 Example 21 94 Example 52 Example 22 92 Example53 Example 23 86 Example 54 Example 24 85 Example 55 Example 25 81Example 56 Example 7 92 Example 57 Example 8 94 Example 58 Example 9 93Example 59 Example 26 65 Example 60 Example 27 83 Example 61 Example 2891 Example 62 Example 29 93 Example 63 Example 30 92 Example 64 Example31 86 Example 65 Example 32 85 Example 66 Example 33 81

TABLE 8 Cycle properties (discharged Battery Separator used capacityretention [%]) Comp. Example 23 Comp. Example 1 80 Comp. Example 24Comp. Example 2 83 Comp. Example 25 Comp. Example 12 76 Comp. Example 26Comp. Example 13 62 Comp. Example 27 Comp. Example 3 80 Comp. Example 28Comp. Example 4 83 Comp. Example 29 Comp. Example 14 74 Comp. Example 30Comp. Example 15 59 Comp. Example 31 Comp. Example 5 80 Comp. Example 32Comp. Example 6 83 Comp. Example 33 Comp. Example 16 64 Comp. Example 34Comp. Example 17 82 Comp. Example 35 Comp. Example 7 60 Comp. Example 36Comp. Example 8 62 Comp. Example 37 Comp. Example 9 65 Comp. Example 38Comp. Example 10 68 Comp. Example 39 Comp. Example 11 78 Comp. Example40 Comp. Example 18 75 Comp. Example 41 Comp. Example 19 76 Comp.Example 42 Comp. Example 20 74 Comp. Example 43 Comp. Example 21 75Comp. Example 44 Comp. Example 22 74

Example 67, Comparative Example 45

By the method below, on the polyolefin microporous membrane obtained inExample 1 and Comparative Example 1, an adhesive porous layer containinga vinylidene fluoride resin is layered to produce a separator fornon-aqueous secondary battery.

A vinylidene fluoride resin having a copolymer composition ofVdF/HFP/CTFE=−92.0/4.5/3.5 (mass ratio) and a weight-averaged molecularweight of 410,000 was dissolved in a mixed solvent of DMAc (organicsolvent): TPG (phase separating agent)=60:40 (mass ratio) such that thepercentage of the resin was 12% by mass to prepare a dope.

This dope was coated on both sides of the polyethylene microporousmembrane. Subsequently, the polyethylene microporous membrane on whichthe dope was coated was immersed in a coagulation bath to coagulate thecoated layer. Here, the composition of the coagulation bath was asfollows: water:DMAc:TPG=50:30:20 (mass ratio). Next, washing with waterand drying were performed. By this, a separator for non-aqueoussecondary battery in which adhesive porous layers are formed on bothsides of the polyolefin microporous membrane was obtained.

For the obtained separator for non-aqueous secondary battery, theadherence thereof was evaluated by the method below.

In the same way as in Example 38, a positive electrode and a negativeelectrode were produced. Between the positive electrode and the negativeelectrode, a separator for non-aqueous secondary battery was sandwichedto be laminated in the following order: the positive electrode/theseparator/the negative electrode. This laminate was thermally compressedat 70° C., 1 MPa for 60 seconds, and subjected to T-type peeling at 150mm/min. by using Tensilon (RTC-1210A, manufactured by ORIENTEC Co.,Ltd.). Setting the area of the peeling surface to 100, when thepercentage of the area on which negative-electrode active material ismoved to the side of the separator was 90% or larger, the adherence wasevaluated as “A”, and when the percentage was smaller than 90%, theadherence was evaluated as “B”. The measured results are shown in Table9.

TABLE 9 Example 67 Comp. Example 45 Polyolefin microporous membrane usedExample 1 Comp. Example 1 Adhesive Film thickness μm 3.4 3.4 porous Unitweight g/m² 2.5 2.5 layer Porosity % 59 58 Complex Film thickness μm15.4 15.4 membrane Unit weight g/m² 9.7 9.8 Porosity % 46 45 Gurleynumber sec/100 cc 300 380 Membrane ohm · cm² 2.4 3.8 resistance Recoveryrate % 99 80 after pressurization Adherence — A B

INDUSTRIAL APPLICABILITY

By the polyolefin microporous membrane of the present invention, evenwhen the polyolefin microporous membrane is complexed with aheat-resistant porous layer, good mechanical strength and shutdownproperties are obtained and electrolyte depletion is prevented bycontrolling the degree of crystallinity and the tie molecule volumefraction. The safety of a non-aqueous secondary battery using thepolyolefin microporous membrane or a separator for non-aqueous secondarybattery including the same is thus secured.

In the following, preferred embodiments of a polyolefin microporousmembrane of the present invention will be shown.

<1> The polyolefin microporous membrane of the present invention isconfigured that a degree of crystallinity is from 60 to 85%, and a tiemolecular volume fraction is from 0.7 to 1.7%.

<2> The polyolefin microporous membrane as described in the above <1>,wherein the tie molecular volume fraction of the polyolefin is from 0.7to 1.5%.

<3> A polyolefin microporous membrane as described in the above <1> or<2>, wherein a number average molecular weight is from 30,000 to 80,000.

<4> A polyolefin microporous membrane as described in any one of theabove <1> to <3>, wherein a number of short chain branches contained in1000 carbon atoms in a main chain of the polyolefin is from 1 to 5.

<5> The polyolefin microporous membrane as described in any one of theabove <1> to <4>, including a polyolefin containing an ultra-highmolecular weight polyethylene having a weight-average molecular weightof 1,000,000 or higher and a high-density polyethylene having a densityof 0.942 g/cm³.<6> The polyolefin microporous membrane as described in any one of theabove <1> to <5>, which is produced by

preparing a polyolefin solution by melt-kneading from 1 to 35 parts bymass of polyolefin and from 65 to 99 parts by mass of mixed solventcomprising a volatile solvent and a nonvolatile solvent at from 190 to220° C.;

forming a gel composition by extruding the polyolefin solution through adie at a temperature from the melting point of the polyolefin to themelting point +60° C. and cooling the extruded polyolefin solution;

removing the volatile solvent from the gel composition;

drawing the gel composition; and

removing the nonvolatile solvent from the gel composition.

<7> A separator for a non-aqueous secondary battery, the separatorcomprising:

the polyolefin microporous membrane as described in any one of the above<1> to <6>; and

a heat resistant porous layer containing a heat resistant resin andprovided on one side or both sides of the polyolefin microporousmembrane.

<8> The separator for non-aqueous secondary battery as described in theabove <7>, wherein the heat resistant resin is at least one resinselected from the group consisting of fully aromatic polyamides,polyimides, polyamide imides, polysulfones, polyketones,polyetherketones, polyether imides and cellulose.<9> The separator for non-aqueous secondary battery as described in theabove <7> or <8>, wherein the heat resistant porous layer furthercontains an inorganic filler.<10> The separator for non-aqueous secondary battery as described in theabove <9>, wherein the inorganic filler is at least one of aluminiumhydroxide or magnesium hydroxide.<11> A separator for a non-aqueous secondary battery, the separatorcomprising:

the polyolefin microporous membrane as described in any one of the above<1> to <6>; and

an adhesive porous layer containing vinylidene fluoride resin providedon one side or both sides of the polyolefin microporous membrane.

<12> The separator for non-aqueous secondary battery as described in theabove <11>, wherein the vinylidene fluoride resin is selected from thegroup consisting of:

(i) polyvinylidene fluoride; and

(ii) a copolymer in which a vinylidene fluoride, and at least one ofhexafluoropropylene, chlorotrifluoroethylene, hexafluoroethylene orethylene are at least copolymerized.

<13> A non-aqueous secondary battery, comprising:

a positive electrode;

a negative electrode; and

the polyolefin microporous membrane as described in any one of the above<1> to <6> or the separator for a non-aqueous secondary battery asdescribed in any one of the above <7> to <12>, which is disposed betweenthe positive electrode and the negative electrode, and wherein anelectromotive force is obtained by doping and dedoping lithium.

<14> A method of producing the polyolefin microporous membrane asdescribed in any one of the above <1> to <6>, the method comprising:

preparing a polyolefin solution by kneading from 1 to 35 parts by massof polyolefin and from 65 to 99 parts by mass of mixed solvent composedof a volatile solvent and a nonvolatile solvent at from 190 to 220° C.(drawing process);

forming a gel composition by extruding the polyolefin solution through adie at a temperature from the melting point of the polyolefin to themelting point +60° C. and cooling the extruded polyolefin solution(extruding process);

removing the volatile solvent from the gel composition (the firstsolvent removal process);

drawing the gel composition (drawing process); and

removing the nonvolatile solvent from the gel composition (the secondsolvent removal process).

The entire disclosures of Japanese Patent Application No. 2010-068117filed on Mar. 24, 2010, Japanese Patent Application No. 2010-068118filed on Mar. 24, 2010 and Japanese Patent Application No. 2010-068119filed on Mar. 24, 2010 are as a whole incorporated herein by reference.

All documents, patent applications and technical specifications recitedin this specification are incorporated herein by reference in thisspecification to the same extent as if each individual publication,patent applications and technical standard was specifically andindividually indicated to be incorporated by reference.

The invention claimed is:
 1. A method of producing a polyolefinmicroporous membrane, wherein a degree of crystallinity is from 60 to85%, and a tie molecule volume fraction is from 0.7 to 1.7%, the methodcomprising: preparing a polyolefin solution by kneading from 1 to 35parts by mass of polyolefin and from 65 to 99 parts by mass of mixedsolvent composed of a volatile solvent and a nonvolatile solvent at from190 to 220° C.; forming a gel composition by extruding the polyolefinsolution through a die at a temperature from the melting point of thepolyolefin to the melting point +60° C. and cooling the extrudedpolyolefin solution; removing the volatile solvent from the gelcomposition; drawing the gel composition; and removing the nonvolatilesolvent from the gel composition.
 2. The method of producing thepolyolefin microporous membrane according to claim 1, wherein the tiemolecule volume fraction of the polyolefin is from 0.7 to 1.5%.
 3. Themethod of producing the polyolefin microporous membrane according toclaim 1, wherein a number average molecular weight is from 30,000 to80,000.
 4. The method of producing the polyolefin microporous membraneaccording to claim 1, wherein a number of short chain branches containedin 1000 carbon atoms in a main chain of the polyolefin is from 1 to 5.5. The method of producing the polyolefin microporous membrane accordingto claim 1, comprising a polyolefin containing an ultra-high molecularweight polyethylene having a weight-average molecular weight of1,000,000 or higher and a high-density polyethylene having a density of0.942 g/cm³.